This invention relates to solid state switching devices and more particularly to triac solid state switching devices.
The term "triac" is a generic term that has been coined to identify a bi-directional three terminal multi-layer semiconductor device which is triggered into conduction by a gate signal (positive or negative) applied to the gate electrode or terminal. Triacs have been used rather extensively for switching of AC power circuits having voltages of up to 240 volts (RMS). Although it is apparently possible to construct triacs that are capable of controlling AC power above 240 volts (RMS), their cost of manufacture increases rapidly, almost exponentially with line voltage.
Generally a triac is triggered from a nonconductive state to a conductive state with the application of a small amount of current either negative or positive applied to the gate terminal. Consequently, rather complicated and elaborate circuits must be devised to make sure that the external signal current applied to the gate terminal is not a transient signal but represents the desired triggering signal. Generally the circuits are devised utilizing either a separate control circuit source for applying the necessary gate current to trigger the triac or a shunt circuit from the applied power line to trigger the triac.
In order for a triac to effectively control the switching it is necessary that the breakover voltage of the triac be higher than the peak voltage applied across the triac. The prior art indicates that a gate current of a specified amplitude of either polarity will trigger the triac into conduction provided that the applied peak voltage is less than the breakover voltage of the triac. If the breakover voltage is exceeded, even transiently, the triac will switch to the conducting state and remain conducting until the current drops below a holding current value. Frequently the turning on of the triac by a transient voltage has extremely undesirable hazardous consequences to the circuit being controlled. Either a circuit must be designed that has a system for transient suppression or the triac itself must be designed so that the triac breakover voltage substantially exceeds the designed peak voltage of the system.
Triacs are commercially available having breakover voltages approaching 500 volts. However, such triacs are unacceptable for use in high voltage alternating power circuits such as 480 volts (RMS) which have peak voltages approaching 700 volts. Consequently, use of a triac rated at 500 volts would be unacceptable for use in controlling or switching a 480 volt (RMS) power circuit.
Triacs that are capable of handling 480 volts (RMS) are many fold more expensive than triacs that are able to switch 240 volt power circuits.
One of the principal objects of this invention is to provide a solid state alternating current switching device utilizing triacs in such a configuration that the use of an external gate current is not required to trigger the triacs into conduction.
An additional object of this invention is to provide a solid state switching device utilizing triacs in which one is able to utilize the device for controlling a high voltage system having a peak voltage that is greater than the breakover voltage of any one of the triacs.
A further object of this invention is to provide a triac solid state switching device having an isolated triggering circuit requiring no external power source to trigger the device into conduction.
A further object of this invention is to provide a high voltage solid state switching device that is extremely reliable and yet inexpensive to manufacture.
A further object of this device is to provide a low cost solid state switching device capable of handling high voltage and high current requirements.
An additional object of this invention is to provide a low cost solid state switching device capable of controlling a high voltage alternating current circuit in which switching is accomplished at or near zero value line voltage.
A still further object of this invention is to provide a solid state power switching device that may be manufactured for a very low cost to reliably switch a high voltage electrical motor operating on 480 volts (RMS).
These and other objects and advantages of this invention will become apparent upon the reading of the following detailed description of preferred and alternate embodiments.